Move existing content around based on the doc-migration specification.

Replace :doc: markup with :ref: to have sphinx keep track of where the
files move and generate valid hyperlinks.

Add a few toctrees and index pages for the new directories.

Depends-On: Ia750cb049c0f53a234ea70ce1f2bbbb7a2aa9454
Change-Id: I253ee8f89d3ec40e39310c18bb87ed1d3d5de330
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

In volume snapshot command, <volume> is the same
as <snapshot-name> when --volume is not specified,
but <volume> cannot be None, so when <snapshot-name>
is not specified (<snapshot-name> is None), a NoneType
error appears.
So make <snapshot-name> no longer optional, it should
be always present.

Change-Id: I3d9f10753a8ef601e70816421c160598e2cc811f
Closes-bug: #1659894

Cleanup help strings and docs for clarity and to keep things consistent:
* --limit metavar should be <num-resource> to indicate what is being counted
* --marker metavar should be <resource> or <resource-id> to indicate the
  type of value being specified
* <*-ip-address> metavars should be just <ip-address> as there is no difference
  in format between fixed and floating IPs
* Move all occurances of '(name or ID)' to end of help text

Change-Id: I2c31746ed6ded3845244e03e57d809f8bc0e6b9d

Supporting "--no-property" option will apply user a convenient
way to clean all properties of volume snapshot in a short command,
and this kind of behavior is the recommended way to devref.
The patch adds "--no-property" option in "volume snapshot set" command,
and update related test cases and devref document.

Change-Id: I5f10cc2b5814553699920c4343995b2e11416e4e
Implements: blueprint allow-overwrite-set-options

Add "--project" and "--project-domain" option to volume v2's
"volume snapshot list" command, it will filter list result by different project.

Change-Id: I7dccd6d8d9f1889fa9cb0c2d04a42d77975c645b

Checked and fix all missing of the review:
https://review.openstack.org/#/c/412669/
https://review.openstack.org/#/c/412673/
https://review.openstack.org/#/c/412678/

Change-Id: Ice85958874121eeb574d0b4cfa58e1590b014ed6

In the current doc, the command examples are like
"os server create" but the acutal command should
be started with "openstack" instead of "os", it
is misleading to first time users.

Change-Id: Ie67c0152d8ff8b7c456f91dc8b9a9164437ee9d1

Add "--remote-source" option to "volume snapshot create" command
to support creating snapshot from an existing remote snapshot in
volume v2 (v2 only), also add the doc, unit tests and release note.

Change-Id: I9e5fad4f0db5b44d528eb6b930edbc816e392c3a
Implements: bp cinder-command-support
Closes-Bug: #1618676
Co-Authored-By: Sheel Rana <ranasheel2000@gmail.com>